(1) Field of the Invention
The present invention relates to the supporting of fuel elements within a nuclear reactor and particularly to the attachment of web members comprised of a first metal to guide tubes comprised of a second metal to define a fuel element supporting spacer grid. More specifically, the present invention relates to high strength nuclear reactor fuel assemblies characterized by the use of different metals to define the fuel element receiving grids and the guide tubes on which the grids are supported and in which neutron absorber elements move for control purposes. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
The functions performed by and the considerations which enter into the design of spacer grids for nuclear reactor fuel assemblies are discussed in detail in U.S. Pat. Nos. 3,607,640 and 3,664,924 issued to Donald M. Krawiec and assigned to the assignee of the present invention. Prior art spacer grids have, in many cases, been fabricated substantially entirely of a zirconium alloy; i.e., zircaloy. The use of annealed zircaloy has been dictated by its desirable combination of mechanical strength, workability and low neutron capture cross-section. Some designers, however, have favored fuel assemblies wherein the guide tubes are comprised of a first metal, for example zircaloy, while the grid defining members are fabricated from a second metal; the second metal typically having a higher neutron capture cross-section when compared to zircaloy but also having a greater stiffness than annealed zircaloy. Typical of the materials employed in spacer grids and having a greater stiffness than zircaloy is the steel alloy known as Inconel.
A fuel assembly for a nuclear reactor will, in most instances, comprise a plurality of guide tubes which extend between upper and lower support plates; the support plates maintaining the requisite parallelism of the guide tubes. Intermediate the support plates, and mounted on the guide tubes, will be a plurality of grids each of which define an "egg crate" type structure. The fuel elements, which typically will comprise zircaloy tubes containing pellets of enriched uranium, are frictionally engaged in the spacer grids and are held in position thereby in parallelism to the guide tubes. As noted above, it is often considered desirable to employ a material such as Inconel to define the spacer grids while utilizing zircaloy guide tubes. Inconel, however, cannot reliably be welded to zircaloy and thus the use of dissimilar materials for the spacer grid and guide tubes precipitates a problem in the mounting of the spacer grids on the guide tubes.
There have, in the prior art, been a number of techniques proposed for the joining of spacer grids comprised of a first metal to guide tubes comprised of a second dissimilar metal. These prior art techniques generally provide for a friction fit and/or mechanical stops between the grids and tubes and have proved to be a less than satisfactory solution since the inherently present clearances between parts have made possible vibration induced movement of the spacer grids relative to the guide tubes with the inherent possibility of fretting the fuel element cladding incident to any such vibration. Restated, the prior art joining techniques have provided only for mechanical capture of the grids on the tubes and have not guaranteed tightness between the parts as is required to insure against relative axial, radial and azimuthal motion. U.S. Pat. No. 3,920,516 discloses a prior art mechanical capture technique for use in mounting Inconel grids on zircaloy guide tubes.